Slack pulling carriages are generally known in the art. Referring to FIG. 1, a diagram of a slack pulling carriage 10 in accordance with the present invention is shown. Though FIG. 1 illustrates a slack pulling carriage incorporating the present invention, there are aspects of the present slack pulling carriage that are common to all such slack pulling carriages and some of these aspects are now discussed as background to facilitate a better understand the present invention.
A slack pulling carriage generally employs a sky line that is steadfastly mounted to a tree stump or other mount at a distal end (not shown) and to a yarder 15 at a landing 25. The carriage 10 moves freely along the sky line 20 by virtue of suitable pulley arrangements 17. A clamp 18 is also provided for securely holding the carriage at a desired position along the sky line. A skidding line 30 feeds through a drum arrangement (not shown) within the carriage and has one end 32 affixed at landing 25 and another end 34 which descends from the carriage. End 32 is attached to a mechanical driver for retracting (or letting out the skidding line) and end 34 is the end to which cut logs are attached for removal from the forest.
The carriage also includes a clamp (not shown) adjacent the drum arrangement for securely clamping the carriage to the skidding line. When this clamp is closed about the skidding line (and the sky line clamp is open), movement of the skidding line causes the carriage to move along sky line 20. When the skidding line clamp is released (and the sky line clamp is closed), movement of the skidding line causes end 34 to be raised or lowered, hence, for example, permitting a log to be lifted from the forest floor.
Carriage 10 also includes a motor for driving the drum arrangement. The combination of the motor and drum arrangement permits movement of the carriage along the skidding line (with both clamps open). The motor, drum arrangement and clamps may be actuated by remote control (often using radio frequency control signals).
With respect to prior art slack pulling carriages, these carriages utilize drum arrangements that are disadvantageous for various reasons (discussed below), particularly in view of current forest practices. For example, one known drum arrangement that utilizes a plurality of drums with the skidding line threaded intricately through the drums. This multiple drum arrangement provides a high degree of friction between the carriage and line which in turn permits secure movement of the line relative to the carriage. The multiple drum arrangement, however, is disadvantageous, in that it (1) induces an undesirable amount of stress on the skidding line and drum components and (2) is large, relatively expensive and heavy (requiring more stringent skyline anchoring and more energy to move). Furthermore, machines with multiple drums may be inappropriately large for current forest practices which are directed to the harvest of second or third growth forests, which provide considerably smaller logs than first growth or virgin forests.
Another known slack pulling carriage utilizes a more simplified drum arrangement in which a single drum is provided. In this arrangement, the skidding line is positioned between the drum and two sheaves. An actuator causes the sheaves to contact the skidding line and push the skidding line into contact with a portion of the drum. A disadvantage of this arrangement, however, is that a drum/skidding line region of sufficient length to achieve efficient and accurate movement of the carriage (i.e., no slipping on the line) is not achieved. Slipping on the line may also tend to disadvantageously accelerate deterioration of the line.